1. Field of the Invention
The present invention relates to a burner for operating a heat generator according to the preamble of claim 1.
2. Discussion of Background
EP-0 780 629 A2 has disclosed a burner which consists of a swirl generator on the incident-flow side, the flow formed herein being passed over smoothly into a mixing section. This is done with the aid of a flow geometry, which is formed at the start of the mixing section for this purpose and consists of transition passages which cover sectors of the end face of the mixing section, in accordance with the number of acting sectional bodies of the swirl generator, and run helically in the direction of flow. On the outflow-side of these transition passages, the mixing section has a number of prefilming bores, which ensure that the flow velocity along the tube wall is increased. This is then followed by a combustion chamber, the transition between the mixing section and the combustion chamber being formed by a jump in cross section, in the plane of which a backflow zone or backflow bubble forms. The swirl intensity in the swirl generator is therefore selected in such a way that the breakdown of the vortex does not take place inside the mixing section but further downstream, as explained above, in the region of the jump in cross section. Here, the swirl generator performs the function of a premix section. The latter consists of at least two hollow, conical sectional bodies which are nested one inside the other in the direction of flow, the respective longitudinal symmetry axes of the individual sectional bodies running mutually offset. As a result, the adjacent walls of the sectional bodies form inflow ducts, tangential in their longitudinal extent, for a combustion-air flow, at least one fuel nozzle acting in the interior space formed by the sectional bodies.
Although this burner, compared with those from the prior art, guarantees a significant improvement with regard to intensification of the flame stability, lower pollutant emissions, lower pulsations, complete burn-out, large operating range, good cross-ignition between the various burners, compact type of construction, improved mixing, etc., it has been found that, when fuels having a lower calorific value, so-called low-calorific fuels, namely MBTU and LBTU gases, are injected through the fuel nozzles along the air-inlet ducts, the gas supply pressure greatly increases, which is reflected in a lower efficiency of the plant, here a gas turbine. Furthermore, since these fuels have high H.sub.2 and CO portions, the flame velocity greatly increases, whereby there is the risk of the flame flashing back into the burner. In such a configuration, the burner changes to a diffusion mode, which then inevitably leads to high NO.sub.x emissions. In addition, there is then the inherent risk that the burner threatens to overheat or that parts thereof may be burnt off. In burners belonging to the prior art, the fuel is therefore injected as far downstream as possible, so that the flame cannot flash back upstream. Here, the fuel is often diluted with steam or with nitrogen, although the efficiency is then reduced in both cases.